I am designing a system using a PIC micro controller chatting thru a
Cermetek CH1786LC modem.

The PIC requests a command thru the modem to a PC. The PC returns a command
of what it wants to do, then the transaction is completed. I want to put a
time out on waiting for the command, does anyone know what the maximum
latency of the phone system is, or what an appropriate time out would be
for this setup? I am limiting connection to V.22bis.

> I am designing a system using a PIC micro controller chatting thru a
> Cermetek CH1786LC modem.
>
> The PIC requests a command thru the modem to a PC. The PC returns a command
> of what it wants to do, then the transaction is completed. I want to put a
> time out on waiting for the command, does anyone know what the maximum
> latency of the phone system is, or what an appropriate time out would be
> for this setup? I am limiting connection to V.22bis.

If you want to be thorough, assume the total path delay to be twice around
the globe in a total of 4 sattelite bounces each. Count each bounce (up &
down) == 80e3 km, so 80e3 * 8 = 640e3 km, or <2.2 seconds. Add to this all
the system delays you can think of and some safety margin. That's it.

Most modem-based systems for local calls implement a timeout of 20 seconds
or more for the shortest timeouts anyway.